Skip full -description for some values in stream names

Fixes #482.

It turns out string formatting isn't our bottleneck – the repeated invocations of `-description` are, mostly against user-provided objects (like `MTLModel`s).

This still isn't the fastest thing in the world, and there are pathological cases (e.g., extremely recursive names like @Coneko points out in #257), but this seems like a good balance of valuable debugging information and execution speed. And, of course, names are still turned off in Release builds.